The present invention relates to medical imaging systems. It finds particular application in conjunction with nuclear or gamma camera systems in which the gantry moves relative to a fixed patient table and will be described with particular reference thereto. However, it is to be appreciated, that the invention will also find application in conjunction with nuclear camera and diagnostic scanners of other types and designs such as PET scanners, digital x-ray equipment, CT scanners, and the like.
Heretofore, nuclear gamma cameras have included a detector head which receives radiation emanating from the patient. The head includes a flat scintillation crystal which converts incident radiation to flashes of light. Internal electronics convert each flash of light into an indication of the location and energy of each received incident radiation event. Typically, the detector head is housed in a radiation blocking material, such as a lead housing, and weighs several hundred pounds.
Various table and gantry systems have been provided which enable one or more detector heads to be positioned at selected locations relative to the patient. For some medical procedures, the head is stationarily positioned over the patient's chest or other examined body area. In other medical procedures, one detector head moves just above and along a longitudinal axis of the patient. When a second head is provided, it generally moves concurrently below the patient and along the longitudinal axis. In this manner, whole body scan information is provided. When a medical procedure is performed which calls for a tomographic type scan, one or more camera heads are rotated around the longitudinal axis. Typically, the detector heads move toward and away from the patient as they rotate, rather than rotating along a circular path, to minimize distance between the detector head and the patient.
Various gantry and table top systems have been provided for facilitating the relative movement of the detector head and the patient, for assuring that the detector heads are at the appropriate, non-canted position relative to the patient, for maintaining tension on cables during detector head movement, and the like.
In one of the prior art gantry systems, each head is mounted on a pair of arms. The arms are centrally pivotally on a large diameter bearing ring with a counterweight at the other end. Although the counterweights reduce the effective weight of the camera head, the length of the cantilevered arms require the gantry to consume a relatively large area. Moreover, moving counterweights tends to be a safety hazard to operator personnel.
Another prior art camera mounts the head pivotally to a single arm with appropriate control mechanisms to move and rotate the arm and head. By eliminating the counterweights, space is saved. However, the large weight of the heads necessitates solid parts and strong drive mechanisms.
The patient support table is typically supported adjacent one end in a cantilever fashion in order to accommodate both longitudinal and circumferential movement of multiple detector heads without the support interfering with head movement. One of the problems with a table top that is cantilevered from one end is that it tends to deflect and oscillate. The maximum deflection for a cantilevered table is described mathematically as WL.sup.3 /8EI, where E is elasticity, I is the moment of inertia, L is the length, and W is the load.
In another prior art system, the head is mounted for longitudinal movement along an elongated horizontal beam. The beam is connected at opposite ends for rotation around the circumference of the patient.
In another prior art system, the patient is received along an axis of a pair of spaced, parallel large diameter bearings. A camera head and a counterweight are connected with the bearings 180.degree. apart. Alternately, a second head is connected with the inner race diametrically opposite the first head.
In such systems, the patient table is either cantilevered or simply supported at opposite ends. When the patient table is simply supported, i.e. supported to permit relative pivotal movement between the table and end supports, the gantry and detector heads are typically moved both longitudinally and rotationally with respect to the patient. The simply supported table deflects much less than the cantilevered table. The maximum deflection for the simply supported table is described mathematically by 5WL.sup.3 /384EI.
The maximum stress for a cantilevered table top is about four times the maximum stress for a simply supported system. In order to minimize deflection and to provide sufficient strength to withstand the stress, simply supported table systems are relatively thick and cantilevered tables are even thicker. The thickness of the table top increased radiation absorption, increases the minimum patient to detector head distance, increases cost, and reduces the amount of radiation received by a detector head disposed below the patient.
The detector heads, being several hundred pounds, tend to become canted during movement. The effects of gravity and the large mass cause supporting posts to shift or deflect, particularly if the head is not supported and driven on both sides. This causes an increase in support structure, more complexity, and a duplication of drives.
Some of the prior art gantry systems are mounted on rails to move relative to a stationary patient. Typically, these gantry systems are supported on three or four wheels. Four or more wheels require precise adjustment of the wheels and precisely flat rail surfaces, lest the gantry rock. One of the problems with a three wheel support system is that the gantry was supported on one side by only a single wheel. The single wheel leads to potential instability and tipping problems.
Cables interconnect each detector head with stationary electronic circuitry. In a dual head camera, a separate take up mechanism is typically provided for handling the cable extending from the heads during rotation. The cable handling system uses a variety of guides, springs, or counterweights for fixing the position and tension of the cables continuously during motion to prevent tangling, kinking, or undue stress on the cables. Depending on the diameter over which the detector heads are rotated, the cable take up mechanism may have several feet more slack at some orientations of the head than others. In one cable take up assembly, each cable passed over a pair of pulleys and had a weight mounted by a pulley therebetween. As the cable lengthened and contracted, the weight carrying pulley moved up and down, analogous to an elevator. This arrangement kept a constant tension on the cables, but required a large area within which the weight mounted pulley could move. A similar assembly was required for the cable from the other head. In other take up mechanisms, the weight was replaced by a spring. The spring enabled the pulley to travel along horizontal or other non-vertical paths, but still required an intended region for accommodating the cable as it was played in and pulled out.
The present invention contemplates a new and improved gantry and table system which overcomes the above-referenced problems and others.